The instant invention resides in the art of antiskid systems and particularly those for aircraft. More specifically, the invention is an AC to DC converter for changing the AC output signal of a wheel speed transducer to a varying DC level indicative of the instantaneous rotational speed of the associated wheel of an aircraft or other vehicle.
It has been previously known in the art to provide a wheel speed transducer or generator with each of the braked wheels of an aircraft to produce an AC signal corresponding to the associated wheel's rotational speed. Most generally, this AC signal is converted to a DC level having an amplitude which varies with the rotational speed of the wheel and, correspondingly, the frequency of the AC signal. This varying DC output is applied to skid detection, locked wheel, feed forward, and other similar control circuitry of an antiskid system to modulate or otherwise control the application and/or release of brake pressure, particularly when the output of the wheel speed transducer is indicative of skidding activity.
Inherent in the prior art of AC to DC converters for use with an antiskid system is a phase lag existing between the DC level and the AC signal responsible for generating that level. Cyclic changes in the frequency of the transducer output are indicative of cyclic changes of wheel speed, such as might occur from fore and aft oscillation of the landing gear of an aircraft. These changes create corresponding changes in the amplitude of the DC output of the converter which appears as a relatively low frequency AC voltage superimposed on the DC output. Prior art converters tend to lag the phase of the superimposed AC output with respect to the associated change in wheel speed. Generally, this phase lag is due to the capacitive means utilized by the converter for either storing charge, reducing ripple, or otherwise acquiring a satisfactory and steady DC signal. Other phase lags are generally present in antiskid systems such as those which occur from the electro-hydraulic servo valve in the development of brake pressure by an electrical signal. In modern aircraft particularly, wherein landing gears are of short length and low mass, the accumulated phase lags have become substantial. Hence, two principal sources of phase lag are present in most antiskid systems: that from the inherent phase lag of the AC to DC converter, and that from the servo valve.
It has been known in the art to use a feed forward circuit to compensate for the above-mentioned phase lags. To be efficient, such feed forward circuit must be designed to compensate for both the inherent phase shift of the converter and the characteristic phase shift of the servo valve. Such a design is not particularly simple since the frequency characteristics of the two phase lags may be substantially different. In order that the feed forward circuit may be designed around the particular characteristics of the servo valve of the associated aircraft, it is most desirable that the phase lag resulting from the converter itself be treated separately and apart from the feed forward circuit, most preferably in the converter itself.